Duelling compasses

[cyrilslugg]

Some advice please! I went out into Langstone Harbour today to 'swing' my new NASA digital compass. I did as the instructions (slowly rotated the boat clockwise thru 360 degrees) and sure enough all seemed to work ok. As a final check I then aligned the boat's heading with a transit which showed on the chart as 55 deg. The digital compass showed my heading as 80 degrees. Both my normal boat and hand bearing compasses confirmed my heading as 55. I then checked a variety of headings between the compasses. The digital agreed with the 2 analog compasses for headings 0, 180, 270 degrees and all points between, but showed differences of 10 - 25 degs on headings between 0 and 90 degs. The 2 'normal' compasses were always in sync and correct to within a few degs of each other. I'm baffled or am I missing something (like a brain)?

 

[bazobeleza]

sounds like your compass has the proverbial crate of beer next to it. try relocating the sebder head

 

[sarabande]

It sounds as if you possibly have the sensor located near a magnetic / electromagnetic object which is inducing local deviation on a particular heading. Perhaps move the senor to another location and try again ?

 

[cyrilslugg]

I thought that - but there's nothing remotely metal/magnetic within a metre.

 

[cyrilslugg]

Since both of you think its local interference, I'll re-look when I'm down at the w/e. I wonder if there could be anything between the internal liner and the hull?

 

[peterb]

We've had similar effects after swinging the compass in an area later found to have local iron concentrations. The fluxgate compass 'compensated' for the local iron, but of course when we moved away it gave us nearly 20 deg of deviation. Make sure that you make your two circles in a place well away from any iron or power cables.

 

[cyrilslugg]

That's really interesting. Due to the tide, I did go quite close (20feet or so) to a couple of cardinal markers - doh! If I cant find any boat bound interference I'll definitely try swinging it out at sea. Thanks

 

[EdWingfield]

As Peterb suggests, are you sure 360 is enough? I've had to do 720 to set mine.

 

[cyrilslugg]

The manual suggested 360 would be OK if the turn was slow enough. Since it went through the countdown process and reset itself as the instructions said it would, I can only presume it was satisfactory.